Synthesis of novel MoWO4 with ZnO nanoflowers on multi-walled carbon nanotubes for counter electrode application in dye-sensitized solar cells

Novel MoWO4 with ZnO nanoflowers was synthesized on multi-walled carbon nanotubes (MW-Z@MWCNTs) through a simple hydrothermal method, and this unique structure was applied as a counter electrode (CE) for dye-sensitized solar cells (DSSC) for the first time. The synergetic effect of ZnO nanoflowers and MoWO4 on MWCNTs was systematically investigated by different techniques. The amount of MWCNTs was optimized to achieve the best DSSC performance. It was found that the 1.5% MW-Z@MWCNTs composite structure had the highest power conversion efficiency of 9.96%, which is greater than that of traditional Pt CE. Therefore, MW-Z@MWCNTs-based CE can be used to replace traditional Pt-based electrodes in the future.

Transition metal oxides nanomaterials such as TeO 2 , Bi 2 O 3 , MoO, WO 4 , BaO, etc., are used for the development of photovoltaic advanced materials, photocatalyst and smart device 9 . Among the metal oxides, tungsten oxide, WO 4 , is n-type semiconductor with a small bandgap of 2.6 eV which has unique thermal, optical, physicchemical, absorbing ability and electrical proper. WO 4 can be used as electron and hole transport layer due to its high carrier mobility, which will help to improve the carrier transport performance in the DSSC device. However, the photovoltaic applications of WO 4 are limited because of its unfavorable conduction band edge position for one-electron reduction of O 2 and hydrogen reduction reactions. This limitation leads to the fast electron-hole recombination rate and the lower photovoltaic and photocatalytic activity. Therefore, in order to prevent this constraint, the research team was interested in the combination of metal oxides and other materials. Gomathi et al. prepared non-toxic Ni-doped MoO 3 nanostructures for CE through a facile hydrothermal route and obtained a power conversion efficiency (PCE) of 8.39% due to the high electrocatalytic activity of MoO 3 . Two-dimensional ZnO nanoflowers also have good visible transmittance, high chemical stability, excellent morphological properties, and high electron mobility (115-155 cm 2 ) 10 . Rotaba Ansir et al. synthesized Pd@C with ZnO nanorods as a photoanode through a microwave treatment and the co-precipitation method and obtained an efficiency of ~ 3.60% due to the reduction in electron-hole recombination and the increase in the number of photo-generated e − /h + 11 .
In recent years, multi-walled carbon nanotubes (MWCNTs), due to their high surface area, excellent chemical stability, and high electrical conductivity, have been used to improve the properties of metal oxides. In 2021, Mahin Mirzaei and Mohammad Bagher Gholiv synthesized functionalized MWCNTs-encapsulated Ni-doped molybdenum diselenide (f-MWCNTs@NiMoSe 2 ) by a hydrothermal route 12  Synthesis of MW-Z@MWCNTs. MoWO 4 was prepared through a simple hydrothermal route 13 . First, 3.29 g of MoO 3 and 0.5 g of WO 3 were dissolved in 70 mL of DI water under vigorous stirring for one hour. Subsequently, HCL (3 M) was slowly dropped in the solution until the value reached about 2. Further, 3.30 g of (NH 4 ) 2 SO 4 was added to the resultant solution, stirred continuously for 30 min before placing into a Teflon-lined autoclave enclosed in a stainless-steel tank, and held at 190 °C for 5 h. Finally, the solution was cooled at room temperature, washed several times with water and ethanol, and dried in the air at 90 °C for 12 h.
In order to ZnO nanoflower powder, 0.7 g of Zn(NO 3 ) 2 ·6H 2 O was dissolved in 75 mL of DI water, and 0.3 g of HMTA was dropped into the resultant solution. Subsequently, 0.9 g of NaOH was dissolved in 40 mL of DI water and added dropwise into the as-prepared solution. The solution was then continuously stirred at 500 rpm for 1 h at room temperature, kept in a Teflon-lined autoclave at 150 °C for 8 h, and finally, washed twice with water and dried at 80 °C overnight 14 .
In order to synthesize MW-Z@MWCNTs, 0.5 g of MWCNTs was added to 75 mL of dimethylformamide (DMF) under sonication for 2 h. Subsequently, 0.25 g of MoWO 4 and 0.3 g of ZnO nanoflower powder were added into the mixture solution and continuously sonicated for 30 min. The resultant solution was kept in a Teflon-lined autoclave at 150 °C for 8 h, then washed several times with water and ethanol, and finally, placed in a hot-air oven at 80 °C for 24 h. The schematic diagram for MW-Z@MWCNTs fabrication is displayed in Fig. 1.
Fabrication of DSSC cell. The Doctor blade method was used to prepare the TiO 2 paste photoanode and the MW-Z@MWCNTs CE with I − /I 3 − pairs of the liquid electrolyte on the FTO substrate. The TiO 2 paste was soaked in a ruthenium (N719) solution overnight, and the MW-Z@MWCNTs CE was deposited on the FTO substrate by the spin coating technique. A Surlyn film (approximately 30 µm) was used to clip the TiO 2 paste photoanode and the MW-Z@MWCNTs CE together in a sandwich-type cell.
Characterization. The phase compositions of the resultant composites were detected by X-ray diffraction analysis (XRD) under Cu-Kα1 radiation at 1.2 kVA (Rigaku, SmartLab). The morphology and surface characteristics of the composite samples were determined by a Fourier-transform scanning electron microscope (FE-SEM; ThermoFisher Scientific, Apreo2, Germany), and their surface elemental composition and oxidation state were analyzed by X-ray photoelectron spectroscopy (XPS; PHI5000 VersaProbe II ULVAC-PHI, Japan) at Synchrotron Light Research Institute (SLRI), Thailand. A monochromatized Al-Kα X-ray source (hγ = 1486.6 eV) was used to excite the samples. Electrochemical impedance spectroscopy (EIS) was conducted in a frequency range from 0.01 to 100 kHz at a bias voltage of 0 V and an amplitude of 10 mV. Current density-voltage (J-V) characteristics were measured by a Keithley 2400 solar simulator under 100 mW/cm 2 illumination.  15 . In the XRD spectra of MoWO 4 , the peak appeared from the (002) plane of WO 3 was stronger than other peaks, and it happened due  www.nature.com/scientificreports/ to the preferred orientation of WO 3 under the effect of Mo atoms (Fig. 2c). It is confirmed from Fig. 2d that the as-prepared MW-Z@MWCNTs were composed of an interconnected structure.
Morphological analysis. The morphology of MW-Z@MWCNTs was identified by FE-SEM and TEM. The morphology and particle size of MWCNTs are displayed in Fig. 3a,b. It is noticeable that MWCNTs consisted of octahedral-shaped overlapping small tubes of 20-30 µm size. It is clear from Fig. 3(c,d) that ZnO nanoflowers contained densely packed nanoneedles. These flower-like clusters were distributed across the surface substrate 16 . The morphology of MW-Z@MWCNTs is presented in Fig. 3(e,f). MW-Z@MWCNTs were found to be clumped together and possessed an indefinite morphology. The internal microstructural arrangement of the as-synthesized samples was determined by TEM (Fig. 4). It is noticeable from Fig. 4a,b that long MWCNTs with 5-50 nm external diameter overlapped each other. Furthermore, the formation of ZnO nanoflowers can be observed in Fig. 4(c,d). In a single nanoflower structure, petal spikes of 600-800 nm size were superficially directed and extended from the center of the flower. In comparison, MoWO 4 had a relatively flatter morphology with a particle size of about 100 nm. Moreover, the black spots on MW-Z@MWCNTs indicate the attachment of ZnO nanoflowers and MoWO 4 MWCNTs walls 17 .
FTIR analysis. FTIR spectra in the wavenumber range of 400-4000 cm −1 were used to determine the chemical bonding state of ZnO nanoflowers, MoWO 4 , and MW-Z@MWCNTs (Fig. 5). In the spectrum of MoWO 4 , Raman spectroscopy. Raman spectroscopy was used to characterize intrinsic defects in ZnO nanoflowers, MoWO 4 , and MW-Z@MWCNTs (Fig. 6). The E 2H -E 2L and A 1T modes at ∼331 and ∼380 cm −1 , respectively, appeared from ZnO nanoflower crystals (line (a)). The peaks at ∼436 and ∼583 cm −1 could be assigned to the E 2H and E 1L phonon modes, respectively. The E 2H mode represents a typical wurtzite crystal structure and reflects a perfect ZnO nanoflower crystal property. In addition, the centered peak of ZnO nanoflower at 569 cm -1 and  XPS analysis. The XPS spectra of MW-Z@MWCNTs are displayed in Fig. 7. The presence of Mo, W, Zn, and O was well detected in MW-Z@MWCNTs (Fig. 7a). Figure 7b presents the high-resolution XPS spectrum of the Mo 3d peak. The binding energies of Mo 3d 3/2 and Mo 3d 5/2 were calculated as 235.0 eV and 231.5 eV, respectively, indicating the existence of Mo atoms in the + 4 oxidation state 23 . Moreover, the peaks at 531.4 eV and 533.1 eV could be assigned to oxygen atoms in MoWO 4 (Fig. 7c). The spin-or-bit doublets at 35.6 eV, 37.6 eV, and 41.5 eV in the W 4f spectrum correspond to the W 4f 7/2 , W 4f 5/2 , and W 5p 3/2 peaks, respectively, indicating the existence of W atoms in the + 6 oxidization state 24 . In Fig. 7e, the binding energies of 1021.70 eV and 1045.78 eV could be assigned to the Zn 2p 3/2 and Zn 2p 1/2 peaks, respectively. N 2 adsorption-desorption isotherm (BET) analysis. N 2 adsorption-desorption measurements were performed to measure the specific surface areas of MW-Z@MWCNTs (Fig. 8). In a typical BET analysis, a measure of the specific surface area (SSA) of MW-Z@MWCNTs is determined from the volume of N 2 gas adsorbed on the MW-Z@MWCNTs. The basics Brunauer, Emmett, and Teller (BET) theory, the most common method used to describe the specific surface area followed the equation: where W is the weight of gas adsorbed, P/P 0 is the relative pressure, Wm is the weight of adsorbate as a monolayer, and C is the BET constant. Type II isothermals with H 3 hysteresis could be attributed to the mesoporous structure of the samples 25 . Moreover, the large specific surface area (30.33 m 2 g -1 ) and pore size (about 30 nm) of MW-Z@ MWCNTs significantly facilitated the access of the electrolyte and allowed rapid charge transfer kinetics to improve the power conversion efficiency of DSSC.

Photovoltaic study of DSSCs with different counter electrodes. The PCE values of different CEs
for DSSC were calculated by a Class AAA solar simulator under AM 1.5G simulated sunlight ( Fig. 9 and Table 1 Electrochemical impedance spectroscopy (EIS) analysis. The Nyquist plots of different CEs were measured to determine the ionic and electronic transport process of the DSSCs as shown in Fig. 10 and Table 1. www.nature.com/scientificreports/ Normally, two apparent semi-circles are detected in the Nyquist plots. The small arc at a high frequency is attributed to the resistance between the counter electrode and electrolyte mediator. The large arc at the mean frequency is associated with charge transfer resistance (R ct ) at the interfaces of MW-Z@MWCNTs with electrolyte and dye molecules. The resistance element R S in the high-frequency region (> 10 5 Hz) is ascribed to the sheet resistance of the FTO substrates. Similar resistance for counter electrode (R CE ), deduced from highfrequency semi-circle, implies that we used the Pt as a reference counter electrode by replacement of MW-Z@ MWCNTs through our experiments. The R ct value of the 1.5% MW-Z@MWCNTs CE was lower than those of other electrodes. The R ct value increased greatly with the decrease of MWCNTs percentage because the addition of MWCNTs significantly improved the charge transfer capability of MW-Z@MWCNTs CE. Furthermore, the 1.5% MW-Z@MWCNTs CE enhanced the electrocatalytic effect of accessible and interconnected pores in ZnO nanoflowers and MWCNTs, allowing more efficient electron transfer between the electrode and the electrolyte to enhance the reduction reaction of the redox couple.

Conclusion
Novel MW-Z@MWCNTs were successfully synthesized by a simple hydrothermal route for CE applications in DSSC. The chemical states, morphologies, and catalytic properties of ZnO nanoflowers, MoWO 4 , and MW-Z@ MWCNTs were analyzed. The 1.5% MW-Z@MWCNTs structure exhibited better I − 3 electrocatalytic activity than Pt CE under the same conditions. The addition of MWCNTs greatly enhanced the electron charge transport capacity and surface area of ZnO nanoflowers. Hence, this unique MW-Z@MWCNTs structure can serve as an efficient CE for Pt-free DSSCs.

Data availability
The data that support the findings of this study are presented in the main text and are available from the corresponding author upon request.